Prostate cancer is the fourth most prevalent cancer in men. In North America and Northern Europe, it is by far the most common cancer in males and is the second leading cause of cancer death in men. In the United States alone, well over 40,000 men die annually of this disease, second only to lung cancer. Despite the magnitude of these figures, there is still no effective treatment for metastatic prostate cancer. Overwhelming clinical evidence shows that human prostate cancer has the propensity to metastasize to bone, and the disease appears to progress inevitably from androgen dependent to androgen refractory status, leading to increased patient mortality.
In spite of considerable research into therapies for the disease, prostate cancer remains difficult to treat. Surgical prostatectomy, radiation therapy, hormone ablation therapy, surgical castration and chemotherapy continue to be the main treatment modalities. Unfortunately, these methods are ineffective in a significant percentage of cases. The age and underlying health of the man, the extent of metastasis, appearance under the microscope, and response of the cancer to initial treatment are important in determining the outcome of the disease and potential treatment. The decision whether or not to treat localized prostate cancer (a tumor that is contained within the prostate) with curative intent is a patient trade-off between the expected beneficial and harmful effects in terms of patient survival and quality of life.
The identification of novel therapeutic targets is essential for improving the current treatment of prostate cancer patients. Recent advances in molecular medicine have increased the interest in tumour-specific cell surface antigens that could serve as targets for various immunotherapeutic or small molecule strategies.
Among the various elements of the immune system, T lymphocytes are probably the most adept to recognize and eliminate cells expressing foreign or tumour-associated antigens. Cytotoxic T Lymphocytes (CTLs) express the CD8 cell surface marker and are specialized at inducing lysis of the target cells with which they react via the perforin/granzyme and/or the Fas/Fas-L pathways. The T-cell receptor (TCR) for antigen of CTLs binds to a molecular complex on the surface of the target cell formed by small peptides (8-11) residues derived from processed foreign or tumour associated antigens, which associate with major histocompatibility complex (MHC) class I molecules.
The other major T-cell subset, helper T lymphocytes (HTLs or T helper cells), is characterized by the expression of CD4 surface marker. The T helper cells recognize slightly larger peptides (11-20 residues), also derived from foreign or tumour associated antigens, but in the context of MHC class II molecules, which are only expressed by specialized antigen presenting cells (APCs) such as B lymphocytes, macrophages and dendritic cells (DCs).
As a consequence of TCR stimulation of naive CTLs and HTLs by peptide/MHC complexes on APCs, the CTLs mature into effector killer cells capable of lysing (tumour) cells that express the corresponding peptide/MHC class I complex. Activated HTLs amplify CTL responses by making the APCs more effective at stimulating the naive CTLs and by producing lymphokines that stimulate the maturation and proliferation of CTLs. The potentiating effect of T helper cells occurs both in secondary lymphoid organs where the immune response is initiated and at the tumor site where CTL responses need to be sustained until the tumour cells are eliminated. Thus, one would predict that vaccines should stimulate both tumour-reactive CTLs and HTLs to generate effective antitumour immunity.
Antigens suitable for immunotherapeutic cancer strategies should be highly expressed in cancer tissues and ideally not in normal adult tissues. Expression in tissues that are dispensable for life, however, may be acceptable.